1. Field of the Invention
The present invention relates to a television signal converting apparatus for converting a television signal of a certain system into that of another system.
2. Related Background Art
For a down-conversion of a television signal of a high definition television system such as so-called hi-vision (hereinafter called HDTV signal) into a television signal for example of NTSC system (hereinafter called NTSCTV signal), there is already proposed a television signal converting apparatus as schematically shown in a block diagram in FIG. 1.
Referring to FIG. 1, the luminance signal Y and two color difference signals P.sub.R, P.sub.B of the HDTV signal are entered from input terminals 100, 101, 102, and are converted by an inverse matrix circuit 103 into three primary color signals of R (red), G (green) and B (blue). These converted R, G and B color signals are respectively converted by A/D converters 104, 105, 106 into digital signals, then subjected to a conversion of aspect ratio by aspect ratio converting circuits 107, 108, 109 from an aspect ratio of 16:9 based on the HDTV signal to 4:3 based on the NTSCTV signal, further subjected to a conversion of the number of scanning lines in the vertical direction by scanning line number converting circuits 111, 112, 113 from 1125 lines based on the HDTV signal to 525 lines based on the NTSCTV signal, and further subjected to a conversion of the field frequency by field frequency converting circuits 114, 115, 116 from a field frequency of 60 Hz based on the HDTV signal to 59.94 Hz based on NTSCTV signal. Thus processed signals are converted again by D/A converters 118, 119, 120 into analog signals, then subjected to a band limitation by low-pass filters (LPF) 121, 122, 123, and entered into an NTSC encoder 124.
On the other hand, the luminance signal Y of the HDTV signal entered from the input terminal 100 is also supplied to a synchronization separation circuit 110, in which the synchronization signal based on the HDTV signal is separated, then converted into a synchronization signal HD/VD based on the NTSC system in a synchronization conversion circuit 117, and is supplied to the above-mentioned NTSC encoder 124.
The NTSC encoder 124 encodes the R, G and B color signals and HD/VD synchronization signal entered thereto, and releases an encoded signal from an output terminal 125.
FIG. 2 is a block diagram showing the details of the NTSC encoder 124 shown in FIG. 1.
Referring to FIG. 2, the entered R, G and B color signals are supplied to a matrix circuit 126 to generate a luminance signal Y and color difference signals I, Q based on the NTSCTV signal system. The luminance signal Y is supplied through a delay line 127 to an addition circuit 134, while the color difference signal I is guided through a delay line 128 and an I signal low-pass filter 129, then balance modulated in an I signal modulator 131 and supplied to said addition circuit 134, and the color difference signal Q is guided through a Q signal low-pass filter 130, then balance modulated by a Q signal modulator 132 and supplied to the addition circuit 134.
On the other hand, the entered synchronization signal HD/VD is supplied to a synchronization signal generator 135 and an oscillator 136 for generating a carrier signal of 3.58 MHz. The synchronization signal generator 135 generates burst flag pulses BFP and a composite synchronization signal Csync. The carrier signal of 3.58 MHz generated by the oscillator 136 is supplied to a burst modulator 133 and a phase shifter 137 of -57.degree.. The burst flag pulses BFP are modulated in said burst modulator 133 by the carrier signal of 3.58 MHz and supplied to the addition circuit 134, to which the composite synchronization signal Csync is also supplied. The carrier signal of 3.58 MHz from the oscillator 136 is further supplied, after phase shifting by the phase shifter 137 of -57.degree., to a phase shifter 138 of -90.degree., and the carrier signals of a mutual phase difference of 90.degree. generated by phase shifts in said phase shifters 137, 138 are utilized in the balance modulation of the color difference signals I, Q in said modulators 131, 132. The addition circuit 134 multiplexes the various signals supplied thereto as explained above, thus generating an NTSCTV signal.
However, the television signal converting apparatus involves drawbacks in that the hardware structure is complex and includes many adjusting points, since the luminance signal Y and color difference signals P.sub.R, P.sub.B of the HDTV signal are once converted by the inverse matrix circuit 103 into R, G and B color signals, then digitized, further subjected to conversions of aspect ratio, number of scanning lines and field frequency, then returned to analog signals and are encoded into the NTSCTV signal.
For example, in the conversion of the number of scanning lines, since the HDTV signal has 1,125 scanning lines while the NTSCTV signal has 525 lines, it is necessary to form 7 scanning lines of the NTSCTV signal from 15 scanning lines of the HDTV signal. Consequently, in the scanning line conversion even with a simple linear interpolation, there is required a capacity of 14H (H being a horizontal scanning period) of the HDTV signal in the line memory to be used, and there will be required three identical circuits in order to process the R, G and B signals. Besides the NTSC encoder 124 of the structure as shown in FIG. 2 requires a considerable amount of hardware for achieving exact encoding, and involves a number of adjusting points because the encoding operation is conducted in the analog state.